The latest generation of retinal implants has shown striking promise in tests involving a handful of blind patients. The implants have enabled many subjects to recognize objects and obstacles and given one person the ability to read large print. Such advances mark a turning point after decades of slow progress. And experts now say that commercial devices may be just a couple of years away.

Retinal implants are designed to replace the function of damaged light-sensing photoreceptor cells in the retina. In particular, they are aimed at treating degenerative diseases such as retinitis pigmentosa and age-related macular degeneration. Using an array of electrodes placed either beneath the retina or on top of it, the devices work by electrically stimulating the remaining cell circuitry in the retina to produce pixel-like sensations of light, called phosphenes, in the visual field.

Peter Walter at the University Eye Clinic at Aachen, who chaired the Artificial Vision symposium in Bonn, Germany, where results from several projects were presented last week, notes that optimistic claims have been made about retinal implants in the past. But he says the success of several long-term studies has given researchers confidence that the remaining challenges are more technological than biological. "Within two or three years we could have products available," Walter says.

Ongoing trials involving one device, the Argus II, a retinal implant developed by Second Sight of Sylmar, CA, have been so promising that the company is already preparing for the market. "We are going to be starting the work to get applications for CE marking in Europe and authorization in the U.S. from the FDA," says Gregoire Cosendai, the company's director of operations for Europe.

In the past it has often been unclear whether the phosphenes seen by patients were due to the implant functioning correctly or to other factors, such as the recovery of photoreceptors triggered by the trauma of surgery--a phenomenon known as the rescue effect. But now that researchers have moved away from acute implantations--implanting and removing the devices during the same surgical procedure--to chronically implanting them, it is possible to test them more rigorously. Such experiments are difficult and time-consuming, but they can establish when the phosphenes are only occurring in the parts of the retina where there are electrodes, says Walter. "If you switch off the device, then this effect disappears," he says.

Trials of the Argus II have shown that some limited vision can be restored to blind patients, helping them to recognize objects and make out doorways or roadsides. The first commercial devices will offer this kind of vision, says Cosendai. The Argus II consists of a small chip containing about 60 stimulating electrodes and a glasses-mounted camera that feeds images and power to the implant via a wireless induction loop.